Step 1

Whisk mustard powder, fish sauce, ½ tsp. demerara sugar, and 1 Tbsp. very hot water in a medium bowl until sugar is dissolved; season with salt and plenty of pepper. Add steak to bowl and turn several times to coat. Let sit while you make the vinaigrette.

Step 2

Whisk chile, lime juice, 2 Tbsp. olive oil, and remaining ½ tsp. demerara sugar in a large bowl; set vinaigrette aside.

Step 3

Heat vegetable oil in a medium skillet, preferably cast iron, over medium-high. Add steak and cook, turning every couple of minutes, until nicely browned and medium-rare, 7–10 minutes. Add butter to skillet, then tilt skillet toward you and use a large spoon to baste steak with foaming butter for a minute longer. Transfer steak to a cutting board and let rest 5 minutes.

Step 4

Meanwhile, add watercress, cucumbers, and mint leaves to bowl with reserved vinaigrette and toss to combine; season salad with salt and pepper. Top with peanuts and drizzle with olive oil.

Step 5

Slice steak and serve with salad.